Matthias Beck

Impulsive terahertz radiation with high electric fields from an amplifier-driven large-area photoconductive antenna

We report on the generation of impulsive terahertz (THz) radiation with 36 kV/cm vacuum electric field (1.5 mW average thermal power) at 250 kHz repetition rate and a high NIR-to-THz conversion efficiency of 2 x 10(-3). This is achieved by photoexciting biased large-area photoconductive emitter with NIR fs pulses of microJ pulse energy. We demonstrate focussing of the THz beam by tailoring the pulse front of the exciting laser beam without any focussing element for the THz beam. A high dynamic range of 10(4) signal-to-noise is obtained with an amplifier based system.

Terahertz emission from lateral photo-Dember currents.

The photo-Dember effect is known as source of impulsive THz radiation after excitation with femtosecond optical pulses. The origin of the emission is the ultrafast separation of electron and holes in strong carrier gradients due to different diffusion coefficients. For a simple semiconductor surface the time dependent polarization is oriented perpendicular to the excited surface which complicates efficient out coupling of THz radiation. We investigate a new scheme for generating strong carrier gradients parallel to the surface. In that case the photo-Dember currents are oriented parallel to the surface and the generated THz radiation can be easily out coupled. This concept can be scaled up so that multiple phase coherent photo-Dember currents contribute to the THz emission. These passive THz emitters reach electric field amplitudes comparable to high-efficiency externally biased photoconductive emitters.

Energy-Gap Dynamics of Superconducting NbN Thin Films Studied by Time-Resolved Terahertz Spectroscopy

Using time-domain terahertz spectroscopy we performed direct studies of the photoinduced suppression and recovery of the superconducting gap in a conventional BCS superconductor NbN. Both processes are found to be strongly temperature and excitation density dependent. The analysis of the data with the established phenomenological Rothwarf-Taylor model enabled us to determine the bare quasiparticle recombination rate, the Cooper pair-breaking rate and the electron-phonon coupling constant, λ=1.1±0.1, which is in excellent agreement with theoretical estimates.

Microfluidic tuning of distributed feedback quantum cascade lasers

In this Letter, we report the tuning of the emission wavelength of a single mode distributed feedback quantum cascade laser by modifying the mode effective refractive index using fluids. A fabrication procedure to encapsulate the devices in polymers for microfluidic delivery is also presented. The integration of microfluidics with semiconductor laser (optofluidics) is promising for new compact and portable lab-on-a-chip applications.

Investigation of coherent acoustic phonons in terahertz quantum cascade laser structures using femtosecond pump-probe spectroscopy

The dynamics of acoustic vibrations in terahertz quantum cascade laser structures (THz-QCLs) is studied by means of femtosecond pump-probe spectroscopy. The phonon modes are characterized by the folding of the acoustic dispersion into an effective reduced Brillouin zone. An accurate identification of this dispersion allows the sample structure and periodicity to be determined with high precision on the order of 0.1%. By temperature tuning the energy of the electronic levels of the system and performing wavelength dependent measurements, we are able to study the impulsive resonant generation and detection of coherent acoustic phonon modes. These results are supported by simulations of the electronic system that well explain the experimental observations. The effects of interface (IF) roughness on coherent acoustic phonon spectra are clearly observed for equal nominal THz-QCL structures but with different interface qualities.

Photoinduced melting of superconductivity in the high- T c superconductor La 2 − x Sr x CuO 4 probed by time-resolved optical and terahertz techniques

The dynamics of depletion and recovery of a superconducting state in La

Terahertz cyclotron resonance spectroscopy of an AlGaN/GaN heterostructure using a high-field pulsed magnet and an asynchronous optical sampling technique

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Energy dependence of the electron-boson coupling strength in the electron-doped cuprate superconductor Pr1.85Ce0.15CuO4-.DELTA.

Sr

Astro-comb calibrator and spectrograph characterization using a turn-key laser frequency comb

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Probing superconducting gap dynamics with THz pulses

CuO